Variations in electrical impedance of the human tissue may be indicative of lesions. For example, U.S. Pat. Nos. 4,291,708 and 4,458,694 and the article, “Breast Cancer Screening by Impedance Measurements,” by G. Piperno et al., Frontiers Med. Biol. Eng., Vol. 2 pp. 111–117, the disclosures of which are incorporated herein by reference, describe systems for determining the impedance between a point of the surface of the human tissue and some reference point on the body of the patient. With the use of a multi-element probe, an impedance map of a tissue such as a breast can be generated. The impedance map, describing variations in impedance within the tissue, can be used for the detection of tumors and especially malignant tumors.
In the above references, the multi-element probe is constructed as a series of flat, conducting elements, mounted onto a glass epoxy or similar base. A conducting wire is connected between each of these elements and a detector circuitry. Impedance measurements between the elements and a remote part of the body are used to determine impedance variations in the breast, using signal processing circuitry.
For optimum functioning of the multi-element probe, two conditions should preferably be met:
i. there should be good conductivity between the probe elements and the human tissue;
ii. cross-talk between different elements should be minimized.
Although commonly used, gels have drawbacks as interfaces. They increase cross-talk and cause build-up of dried up gel between the elements.
An additional concern is hygiene. It would be desirous for each patient to come in direct contact with a fresh surface, in order to reduce possible bacterial or viral transmission.
U.S. Pat. No. 5,810,742, “Tissue Characterization Based on Impedance Images and on Impedance Measurements,” the disclosure of which is incorporated herein by reference, describes in a preferred embodiment thereof a disposable multi-element probe for the identification of tissue type from impedance maps. The disposable multi-element probe comprises a plurality of sensing elements, preferably separated by dividing elements to eliminate cross talk. The sensing elements comprise a bio-compatible material such as hydrogel filling wells formed by the dividing elements and a mylar substrate or some other flexible, nonconductive substrate. The substrate is pierced in the center of each well. The hole resulting from the piercing is filled with a conductive material which is also present on the bottom of the well and on the backside of the substrate in order to form a pair of electrical contacts on either side of the substrate and an electrically conducting feed-through between the pair of contacts. A separate contact pair and feed-through is provided for each sensing element.
The multi-element probe is used for only one patient and then discarded. The probe is attached to a probe holder which comprises a printed circuit board having a plurality of contacts which correspond to the contacts on the back side of the substrate.
In the above system, each conductive element comprises:
a. a stationary contact on the probe holder;
b. a pair of electrical contacts on either side of the substrate;
c. a conducting feed-through between the pair of electrical contacts; and
d. hydrogel that fills the well between dividing elements.
Since every layer and interface has its own impedance, the plurality of conductive materials and interfaces between them decreases the accuracy of the measurements; a design of many layers reduces the quality of the image. Furthermore, accurate registration is required between the probe and the disposable structure. This is difficult when the spacing is small.
Furthermore, a disposable multi-element probe is rather costly. It would be desirous to have a disposable impedance-mapping system which is less costly and which comprises fewer conductive layers.